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* Did Darwin Really Say That?

When a new theory or hypothesis is put forward in science, we presume that it will be associated with evidence. This is a hallmark of scientific reasoning--theories or hypotheses should be explicitly supported by empirical evidence. It is one of the reasons for our ever increasing confidence in the theory of evolution. From its original presentation to its current incarnations, the supporting empirical evidence has gone from strength to strength, though not equally on all fronts and certainly with a good dose of controversy. Thus, imagine the surprise when readers of Charles Darwin's On the Origin of Species encounter the following passage under the heading "Illustrations of the Action of Natural Selection": "In order to make clear how, as I believe, natural selection acts, I must beg permission to give one or two imaginary illustrations" (Darwin, 1964 [1859]: p. 90). How can "imaginary illustrations" be evidence for the process of natural selection, which is supposed to explain the evolutionary emergence of exquisite adaptations in different species? Every time I introduce undergraduates to On the Origin of Species, this section induces conceptual vertigo. Many of them are perplexed over how this can be a legitimate form of scientific reasoning: maybe Darwin made a mistake or didn't have a choice? As one of my students recently phrased it: "I'm curious about how Darwin used 'reasoning' and invisible thought experiments to explain things.. .don't you always need proof?" Is Darwin really using these imaginary illustrations as evidence for how natural selection acts or is something else going on?

It turns out that answering the question of what Darwin was doing with "imaginary illustrations" illuminates both the meaning of evolutionary concepts and the nature of science. My aim here is to bring this less familiar component of Darwin's reasoning into the context of teaching biology. His imaginary illustrations offer an accessible and novel way to teach about natural selection and scientific reasoning, especially because they demonstrate an underappreciated aspect of scientific theory testing--evaluating explanatory potential with thought experiments. I begin by exhibiting Darwin's use of imaginary illustrations at various places in On the Origin of Species, which displays how they operate as hypothetical reasoning or thought experiments rather than as attempts to confirm his theory with evidence. Next, I show how Darwin's thought experiments are adaptations of a pattern of reasoning used by Charles Lyell that conforms to the philosophical criteria of causal explanation put forward by John Herschel. Having set Darwin's hypothetical reasoning in the context of this methodology, I detail the intriguing ways in which it yields new materials for teaching biology and aligns with the standards articulated in the National Science Education Standards (National Research Council, 1996). In closing, I emphasize how paying attention to Darwin's thought experiments offers a powerful route to teaching the current explanatory power of evolutionary theory.

* Darwin's Reasoning: A Closer Look

The first imaginary illustration deployed by Darwin in On the Origin of Species is about the hunting abilities of wolves (Figure 1):

Let us take the case of the wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that the fleetest prey a deer for instance, had from any change in the country increased in numbers, or that other prey had decreased in numbers, during that season of the year when the wolf is hardest pressed for food. I can under such circumstances see no reason to doubt that the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected,--provided always that they retained strength to master their prey at this or at some other period of the year, when they might be compelled to prey on other animals. …

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